1. Field of the Invention
The present invention relates to a method for the production of a photographic coating support material.
2. Brief Description of the Background of the Invention Including Prior Art
Photographic coating support materials comprise for example photographic base papers with synthetic resin coatings. These synthetic resin coatings are preferably applied on two sides of the base paper and usually comprise a clear polyolefin coating on the back side and a polyolefin coating filled with white pigments on the front side of the base paper. The front side of the base paper is that side which is coated with light-sensitive photographic emulsions during a later processing step.
The synthetic resin coatings are applied according to the melt extrusion process onto the base paper.
Photographic coating support materials of this kind have to be appropriate and suitable for meeting high quality requirements, in particular with respect to the surface properties provided. These surface properties are determined essentially by the synthetic resin coatings, where the synthetic resin coatings have to be as far as possible without fault and free of defects. This requirement is associated with the condition that also the starting materials, which form the synthetic resin coating mass, are to be of a sufficiently high and of an utmost suitable quality.
The essential starting product for providing such a coating is the synthetic resin itself, where the resin is usually a polyolefin, in particular polyethylene.
High-density polyethylene HDPE, low-density polyethylene LDPE, and linear low-density polyethylene LLDPE are employed as polyethylene.
In case of pigment-containing coatings, additionally so-called master batches are admixed. A master batch is a preproduced mixture of high homogeneity of pigment and synthetic resin. White pigments and color pigments are used as pigments. The white pigment employed in connection with photographic coating support materials is predominantly titanium dioxide. The titanium dioxide is present in the master batch in a concentration of generally 50 weight-percent.
Synthetic resins and master batches are available commercially as granules, pellets, or chips.
Synthetic resins and master batches employed according to the present invention are polyolefins and polyolefin master batches. Master batches are premanufactured, well dispersed mixtures of the synthetic resin with white pigments, coloring pigments or dye-stuff or, for example, optical brighteners. Depending on the kind of these pigments, the pigment content can amount of from 0.02 to 76 weight-percent, and preferably from 20 to 50 weight-percent.
Master batches are premelted compositions which are formed to granules, i.e. chips, grains, or pellets, for a better handling.
The melt extrusion in the coating extruder is performed at temperatures between 250.degree. C. and 350.degree. C. Low-molecular components and water are transferred into a gaseous state at these temperatures despite the increased pressure prevailing in the extruder and the low-molecular components and water leave defect locations in the coating after the melt film exits from the T-die of the coating extruder.
Small amounts of humidity result in the so-called water braids. Water braids are localized zones of alternating shiny and matte areas. If the content in humidity is somewhat higher, then the so-called lacing occurs. The term "lacing" denotes holes and slits, respectively, in the extruded film caused by water vapor bubbles. However, lacing can also be caused by low-molecular components with high vapor pressure. The presence of water braids and of lacing results in unusable material.
Less volatile low-molecular components of the coating mass adhere to the lips of the T-die and the less volatile low-molecular components accumulate at the lips of the T-die, and cause then narrow and sharply delineated scratches in the melt film, so-called die stripes. Die strips can be light or dark longitudinal stripes caused by microscratches in the surface of the melt film. The die lips thereupon have to be cleaned, which results in each case in an interruption of the production.
In order to be able to remove and eliminate these interfering materials already prior to the extrusion coating, the starting materials are dried at a temperature of about 80.degree. C. for a time period of up to 24 hours. The temperature and the drying time cannot be increased without limits, because otherwise new problems would occur based on thermal damaging of the polyethylene.
During the drying of master batches with titanium dioxide, very strong degradation and decomposition reactions can occur depending on the drying time, because titanium dioxide promotes this degradation and decomposition. A degradation of the polymer occurs already at a temperature of 80.degree. C. and a drying time from about 11/2 to 2 days depending on the type of titanium dioxide. This effect is particularly pronounced in connection with anatase and is observed predominately when high anatase concentrations of more than 10 weight-percent are present in the final product. In case of lower concentrations of anatase, the polymer decomposition and degradation is in fact also present but is not always recognized.
It has been found that the predrying of the starting products does not suffice in each case in order to be able to produce without troubles and interferences. This holds in particular for pigmented synthetic resin coatings and in particular in cases, where the pigment employed in the coating mass is subjected to a post-treatment with aluminum hydroxide, silicon dioxide, and other hygroscopic substances such as it is conventional in connection with titanium dioxide.
Volatile low-molecular components are entered mainly with the plastic into the system. These components are predominantly auxiliary dispersing agents, separating agents, and lubricating agents, for example, metal stearate, polyglycols, waxes.
The Japanese Printed Patent JP 60,089,310 teaches a method for feeding synthetic resin granules into a dryer connected with a vacuum source and a heat source and for making the inside of the dryer vacuum while heating from the periphery of the dryer.